A light-emitting diode (LED) driving device described in JP 2012-244137 A (hereinafter referred to as Document 1) represents as a conventional example of a lighting device. The light-emitting diode driving device (hereinafter referred to as a conventional example) includes a rectifier circuit, an LED unit, a constant current circuit for charging a capacitor (charging circuit), a constant current circuit for discharging a capacitor (discharging circuit), a charging diode, a discharging diode, and a charging-discharging capacitor. The conventional example is, for example, electrically connected to an AC power supply with an effective value of 100 V, and is configured to rectify an AC voltage of the AC power supply with a rectifier circuit, and to obtain a pulsating voltage with a peak value of approximately 141 V.
A first end of the charging-discharging capacitor and a first end of the discharging circuit are electrically connected to a high potential-side output terminal of the rectifier circuit, and a low potential-side output terminal thereof is electrically connected to ground. An anode of the charging diode and a cathode of the discharging diode are electrically connected to a second end of the charging-discharging capacitor. A cathode of the charging diode is electrically connected to a second end of the discharging circuit and an anode-side terminal of the LED unit. A cathode of the LED unit is electrically connected to an anode of the discharging diode and a first end of the charging circuit. A second end of the charging circuit is electrically connected to ground.
Next, operations of this conventional example will be described.
First, charging of the charging-discharging capacitor is performed for a period during which a power supply voltage of the AC power supply is high. A charging current flows in a path (hereinafter referred to as a charging path) that passes from the rectifier circuit through the charging-discharging capacitor, the charging diode, the LED unit, and the charging circuit in this order, and charges the discharging-discharging capacitor. The charging current is controlled to a constant current by the charging circuit. At this time, the LED unit and the charging-discharging capacitor are connected in series, and loss in the charging circuit can be mitigated due to a charged voltage of the charging-discharging capacitor due to a charged voltage of the charging-discharging capacitor, even if a forward voltage of the LED unit is small and a voltage difference thereof to the power supply voltage is large. Also, the charged voltage of the charging-discharging capacitor is a voltage obtained by subtracting the forward voltage of the LED unit from the power supply voltage at the end of charging. When the charging ends, the current flowing in the charging circuit decreases rapidly, and the discharging circuit starts operation in response to a signal generated when this rapid decrease is detected.
Discharging of the charging-discharging capacitor is performed for a period during which the power supply voltage of the AC power supply is low. The discharge current flows in a path (hereinafter referred to as a discharging path) that passes from the charging-discharging capacitor through the discharging circuit, the LED unit, the discharging diode, and charging-discharging capacitor in this order. Note that the discharge current is controlled to a constant current by the discharging circuit.
Here, a period during which the power supply voltage is higher than the voltage (charged voltage) across the charging-discharging capacitor exists before transitioning from the charging period to the discharging period, and a current flows in the period (hereinafter referred to as a transient period) in a path (hereinafter referred to as a transient path) that passes from the rectifier circuit through the discharging circuit, the LED unit, and the charging-discharging circuit in this order. Note that the current (hereinafter referred to as a transient current) is controlled to a constant current having a current value that is equal to the value of whichever current is smaller between the current in the discharging circuit and the current in the charging circuit (current in the discharging circuit, for example).
According to the conventional example, as described above, the LED unit can be directly driven (lighted) by the pulsating voltage that results from rectification by the rectifier circuit, without the AC electric power supplied from the AC power supply being converted to DC electric power. Moreover, in this conventional example, lighting of the LED unit and charging of the charging-discharging capacitor are performed at the same time by connecting the LED unit and the charging-discharging capacitor in series, for a period during which the pulsating voltage is high, and the LED unit can be lighted by discharging the charging-discharging capacitor for a period during which the pulsating voltage is low. As a result, since there is no period during which the light source (LED unit) is turned off in one cycle of the power supply voltage, flicking can be suppressed.
Incidentally, the conventional example described in Document 1 includes a plurality of LED units connected in series to improve efficiency (power supply efficiency). However, if the number of LED units is increased to improve the efficiency, changing (light ripple) of all light fluxes in one cycle of the pulsating voltage may be increased. Also, if smoothing capacitors are respectively connected in parallel to the plurality of LED units to suppress the light ripple, the cost for manufacturing the lighting device may be increased and the lighting device may be large.